When exposed to freezing conditions, hydrogen fuel cell components that contain reactant fluids and water may experience operating issues due to ice that forms during or after an initial startup of the fuel cell.
For an ejector-based fuel cell system with a passive recirculation loop on the anode side, water present in the recirculation stream may freeze when the system begins operating in cold ambient temperatures or during a cold purge cycle prior to start up. For example, this water may freeze when it initially comes into the ejector and contacts cold surfaces. Alternatively, this water may freeze moments later in the mixing chamber when meeting the incoming stream of cold fresh hydrogen flowing from the primary ejector inlet. For a fuel cell application in a vehicle, the temperature of the fresh hydrogen is largely dependent on the temperature of the fuel storage tank, which likely corresponds with the ambient temperature. Additionally, the mixture of fresh and recirculated hydrogen flowing through the ejector is further cooled as it is expanded across the diffuser, presenting another opportunity for water to freeze.
Ejectors are momentum transfer devices, and as such, the passive recirculation flow they induce is a function of compression work performed by the ejector. Any pressure drops within the anode loop increase the compression work for the ejector, and may limit the recirculation flow. The dominant pressure drop in the anode loop is the fuel cell stack itself, and pressure drops across other components need to be minimized for the ejector and fuel cell to function properly. Therefore ice formation anywhere in the anode loop, including the ejector, represents additional pressure drop that may cause degradation of fuel cell performance until latent heat from fuel cell operation causes the ice to passively melt.
For a fuel cell application in a vehicle, the fuel cell needs to operate in freezing temperatures that the vehicle may encounter. The vehicle and fuel cell may be exposed to temperatures of −25 Celsius or even lower, which is well below the freezing point of water. Cold weather operating issues need to be addressed for a fuel cell vehicle to operate in climates with extreme ambient temperatures, and to meet user expectations for the vehicle.